1. Field of the Invention
The present invention relates to a process for manufacture of a semiconductor device and, more particularly, to a chemical vapor deposition process (normal pressure CVD process) in which an organic source is caused to react with ozone under normal pressure.
2. Description of the Prior Art
Recently, semiconductor devices are generally oriented toward greater compactness and a higher degree of integration and accordingly there has arisen the need for multi-layer wiring. In order to meet such need, as a promising interlayer insulation film forming method, a so-called TEOS (tetra-ethoxy-silane)-O.sub.3 (ozone) system normal pressure CVD process has been receiving attention which enables films to be formed at a low temperature (on the order of 400.degree. C.) and provides a high degree of flat covering. The TEOS-O.sub.3 system normal pressure CVD process is a process in which TEOS (as, for example, bubbled with N.sub.2 gas) and O.sub.3 (with O.sub.2 used a carrier gas) are conducted onto a substrate kept at a predetermined temperature and are caused to react with each other under normal pressure so that silicon oxide is deposited on the substrate.
However, with the above described normal pressure CVD process of the conventional TEOS-O.sub.3 system, there is a problem that since the deposited film contains 1 to 3 wt % of hydrogen in terms of moisture content (which is present in the state of Si--OH), the film is subjected to quality deterioration or ohmic contact between multi-layer conductor lines is likely to be interfered. This adversely affects the reliability of the semiconductor device. The above noted hydrogen content may be somewhat reduced by heat treatment at a temperature of not less than 700.degree. C. Where an inter-layer insulating film is to be formed between metal conductor layers, however, heat treatment at such a high temperature cannot be carried out from the standpoint of metal reliability maintenance.
Further, the TEOS-O.sub.3 system normal pressure CVD process involves the problem that the growth rate of silicon oxide depends upon the material of the substrate. More specifically, where the substrate is made of single-crystal Si, poly Si, or metal, the rate of such growth is higher, while the growth rate is lower where the substrate is made of plasma SiO.sub.2 or thermal (thermally oxidized) SiO.sub.2 (and a median growth rate prevails where the substrate is of SiN). FIG. 8 illustrates by way of example how growth rate differs between poly Si and thermal SiO.sub.2 when respectively used as substrates (.quadrature. represents rate of growth on the poly Si substrate, and .largecircle. represents rate of growth on the thermal SiO.sub.2 substrate). Since the rate of growth varies in this way depending upon the kind of substrate material, when deposition is made over lines of metal wiring laid at a predetermined pitch on an insulating film (i.e., at a metal layer covering stage), step coverage may be so unfavorable that no good surface smoothness can be achieved.
In order to avoid this problem, an attempt has been made such that at the metal layer covering stage, a silicon oxide film (plasma TEOS film) is first made from TEOS by the plasma CVD process and thereafter a silicon oxide film is formed by the TEOS-O.sub.3 system normal pressure CVD process. However, this attempt involves an increase in the number of stages and, in addition, has a disadvantage that at a sub-halfmicron zone, the step coverage of the plasma TEOS film per se is so unfavorable that no good surface smoothness can be obtained.
In the TEOS-O.sub.3 system normal pressure CVD process, the dependence of film growth rate may be overcome by decreasing the ozone concentration to about 0.5 to 1.5 wt %.
However, this will result in increased film shrinkage by annealing and thus adversely affect the quality of the silicon oxide film formed.